Conventionally, in a communication device such as a mobile phone, a filtering device has been used to separate signals having different bands, such as a transmission signal and a reception signal, for example. Bulk acoustic wave (BAW) resonators have been used for such filtering devices. The BAW resonators may include film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs) formed by multiple acoustic films.
FIG. 1 is a circuit diagram showing an example of a conventional ladder-type bandpass filter formed by FBARs. A ladder-type filter 110 includes series-connected FBARs 111, 113, 115, 117 connected in series along a signal path extending from an input (IN) to an output (OUT). The ladder-type filter 110 further includes parallel-connected FBARs 112, 114, 116 connected in parallel between respective nodes interconnecting the series-connected FBARs 111, 113, 115, 117 and ground.
FIGS. 2A and 2B illustrate a configuration of a conventional electronic device in which the ladder-type filter 110 of FIG. 1 is formed. FIG. 2A is a plan view of the electronic device 100 and FIG. 2B is an end view of the electronic device. The plan view of FIG. 2A corresponds to a cross section taken along line II-II′ in FIG. 2B.
In the electronic device 100, series-connected FBARs 111, 113, 115, 117 are disposed in series along a conductive signal path 131 extending from a first signal contact 141 to a second signal contact 143 on the top surface 151a of a piezoelectric substrate 151. The piezoelectric substrate 151 is supported by a support substrate 152. Further, parallel-connected FBARs 112, 114, 116 are connected in parallel between a ground plane 132 and respective nodes interconnecting the series-connected FBARs 111, 113, 115, 117 along the conductive signal path 131. The ground plane 132 connects the parallel-connected FBARS 112, 114, 116 to ground contacts 142, 144, 146.
The electronic device 100 includes a sealing substrate 153, which is disposed to oppose the top surface 151a of the piezoelectric substrate 151 via a certain gap. A side wall 154 formed along the outer periphery on the top surface 151a of the piezoelectric substrate 151 spaces the sealing substrate 153 apart from the piezoelectric substrate 151 and encloses the gap between the piezoelectric substrate 151 and the sealing substrate 153. As shown in the end view of FIG. 2B, external electrodes 155, 156 are disposed on the sealing substrate 153. The external electrodes 155, 156 are connected to suitable signal contacts 141, 143 or to ground contacts 142, 144, 146, 148 by vias (not shown), for example.